1. Field of the Invention
This invention relates to a new and improved chemical process for preparing 1,1-dihalo-4-methyl-1,3-pentadienes, key intermediates in a known method for the production of certain pyrethriod insecticides, and to new compositions of matter useful in the practice of the chemical process.
2. Description of the Prior Art
Pyrethriods, naturally-occurring and synthetic derivatives of cyclopropanecarboxylic acid, have long been of interest as insecticides because they are active against a wide range of insect species, they display relatively low toxicity toward mammals, and they do not leave harmful residues. A notable recent technical advance in the pyrethroid art was the discovery of dihalovinylcyclopropanecarboxylates; for example, 3-phenoxybenzyl 2-(.beta.,.beta.-dihalovinyl)-3,3-dimethylcyclopropanecarboxylates, having an outstanding combination of insecticidal properties [Elliott et al., Nature, 244,456(1973); ibid., 246, 169(1973); Belgian Pat. No. 800,006]. Since Elliott's discovery, a great deal of interest has been generated worldwide in economical processes for the production of this type of pyrethroid.
Several years before Elliott's discovery, a method for synthesizing ethyl 2-(.beta.,.beta.-dichlorovinyl)-3,3-dimethylcyclopropanecarboxylate was reported [Farkas et al., Coll. Czech. Chem. Comm., 24, 2230(1959)]. The ethyl ester of Farkas leads to an Elliott pyrethroid by subsequent reaction with 3-phenoxybenzyl alcohol [Nature, 244,456(1973)]. According to the first step of the Farkas method, chloral may be condensed with either isobutenyl magnesium bromide or with isobutylene, using a free radical catalyst with the latter, to produce pentenols which then may be reacted as follows, yielding the cyclopropanecarboxylate in four additional steps: ##STR1##
However, the overall conversion of readily available isobutylene to 1,1-dichloro-4-methyl-1,3-pentadiene, the key reactant in the fourth, ring-closing step of the Farkas method is reportedly less than 40%. Furthermore, for every kilogram of dichloropentadiene produced in the fourth step, more than a kilogram of zinc dust is consumed in the third step. In a recent year, U.S. producers alone sold about 300 million kilograms of synthetic organic insecticides [Chemical and Engineering News, July 28, 1975, p. 19]. If the Elliott pyrethroid becomes a major agricultural commodity, commercial production of this key intermediate by the Farkas method would seriously tax the world supply of zinc. Thus, other more practical and economical processes capable of yielding 1,1-dichloro-4-methyl-1,3-pentadiene from readily available starting materials have been sought.